1. Technical Field of the Invention
The present invention relates to a gas sensor element which detects a concentration of a specified gas to be measured and a gas sensor including the gas sensor element.
2. Related Art
Conventionally, a gas sensor element is known which includes a solid electrolytic substance having a bottomed cylindrical shape and oxygen ion conductivity, a reference electrode arranged on the inner side surface of the solid electrolytic substance, a measuring electrode arranged on the outer side surface of the solid electrolytic substance, and a protective layer which covers the outer side surface of the solid electrolytic substance together with the measuring electrode and allows gas to be measured to pass through the protective layer. (For example, refer to Japanese Patent Application Laid-open No. 2006-38496.)
Such a gas sensor element is formed of a leg portion whose profile line is straight on an axial cross section, which is parallel to the axis of the gas sensor element, and a bottom portion whose profile line is curved.
The above gas sensor element has problems as described below.
Since the bottom portion of the gas sensor element has low water resistance and strength, the bottom portion of the solid electrolytic substance is easily cracked when getting wet. The following reasons are assumed. First, the bottom portion of the solid electrolytic substance has a curved shape as described above, and thermal stress is easily concentrated in the bottom portion. Therefore, the bottom portion is easily cracked when getting wet. Second, water condensed when the car is stopping collects on the corners of a cover of the gas sensor element. The collected water easily contacts the bottom portion of the gas sensor element when the engine starts.
To solve the above problems, increasing the film thickness of the protective layer is assumed to improve the water resistance of the gas sensor element.
However, when the film thickness of the protective layer is simply increased, the following problems can arise. Since increasing the film thickness of the protective layer increases the amount of materials used for forming the protective layer and requires much time to spray the materials, the cost of manufacturing the gas sensor element can rise. In addition, when simply increasing the film thickness of the protective layer, rich components and lean components in the gas to be measured diffuse slowly to the measuring electrode. This can lower responsiveness of the gas sensor element.